This invention relates generally to X-ray tubes and, more particularly, to X-ray tube anodes having a focal track with a limited radial dimension for purposes of reducing off-focal spot radiation while maintaining a constant focal spot size.
X-ray tube targets are conventionally comprised of a relatively low-density substrate, such as molybdenum, with a high-density refractory metal focal track disposed thereon in the form of an annular ring. The associated cathode is then disposed in such a position as to emit electrons for bombardment of the focal track to produce X-rays. The radial width of the focal track is conventionally made sufficiently large so as to overlap on both sides of the electron beam. In this way, the relative alignment between the cathode and the anode is not critical in that, as long as the electron beam is located somewhere on the focal track, the resulting focal spot will be of a fixed size.
One of the problems associated with conventional X-ray targets is that of off-focal spot radiation, the primary cause of which is the straying of so-called "leakage" of electrons from the electron beam. This problem is substantially alleviated by the use of a hooded anode or some other collimation means to provide a fixed channel for the flow of electrons. There is, however, additional structural complications and cost involved with this solution.
Another cause of off-focal spot radiation is that of radiation caused by secondary electrons. As the electron beam bombards the focal track within a prescribed radial area, there are, in addition to the X-rays given off, the generation of secondary electrons which tend to dispurse strike other areas of the focal track, outside of the prescribed radial boundary. When this occurs, X-rays are generated at locations outside of the radial boundary to thereby constitute off-focal spot radiation and resultant reduction in resolution.
One approach for reducing the off-focal spot radiation would be to limit the radial width of the focal track to the same radial width as the projected electron beam. Such a structure is shown in U.S. Pat. No. 3,795,832.
A disadvantage of having equal radial widths for the focal track and the electron beam is that any relative misalignment will result in a focal spot of reduced size. Such a misalignment may result from a deviation of the electron beam, a condition which is substantially controllable by some type of focusing device, such as a cathode cup. Another cause of misalignment and one which is virtually always present, is that of Total Indicated Runout (TIR). This is the phenomenon wherein the radial distance between the center of rotation and the edge of the focal track varies as the anode rotates, thereby causing the focal track to effectively wobble with respect to the electron beam. Inasmuch as there will inherently be some TIR, an X-ray tube having equal radial widths for the electron beam and the focal track will result in a focal spot which varies cyclically in size.
A third and most prevalent cause of misalignment is that of radially mispositioning the filament such that the emitted electron beam is not properly aligned with the focal track.
It is therefore an object of the present invention to provide an X-ray tube with reduced off-focal spot radiation.
Another object of the present invention is the provision in an X-ray tube for a reduction of off-focal spot radiation without an associated variance in the focal spot size.
Yet another object of the present invention is the provision for an X-ray tube which is economical to manufacture and practical to use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.